System and method for determining the maximum output voltage swing on operational amplifiers

ABSTRACT

An operational amplifier is rapidly driven through a voltage swing to a maximum value; that is, the excursion of the output voltage attains a maximum value with a prescribed difference between the input and the output voltages. Initially, a voltage equal to the prescribed voltage is fed through an adder feedback circuit to the input of the amplifier and the resulting output voltage is added to the initial voltage to initiate a regenerative build up of the voltage applied to the input. The output voltage instantaneously increases until the prescribed difference exists between the input and the output voltages. A reading of this voltage indicates the maximum voltage swing at the prescribed difference between the input and output voltages.

I United States Patent [151 3, Stetzler [451 Nov. 14, 1972 SYSTEM'ANDMETHOD FOR 2,506,365 5/1950 Knight ..330/1l2 X DETERlVIINING THE MAXIMUMOUTPUT VOLTAGE SWING ON Primary Examiner-Nathan Kaufman OPERATIONALAMPLIFIERS Attorney-W. M. Kain and R. P. Miller 7 Y 2] Inventor GrantFStetzler, Temple, Pa. [57] ABSTRACT 73 A West E] t sslgnee s g: 35 5;???Incor An operational amplifier is rapidly driven through a voltage swingto a maximum value; that is, the excur- Filedi 1971 sion of the outputvoltage attains a maximum value [211 App]. NO: 118,409 with a prescribeddifference between the input and the output voltages. Initially, avoltage equal to the prescribed voltage is fed through an adder feedback[52] US. Cl. ..330/2, 330/9, 330/100, circuit to the input of theamplifier and the resulting 330/69, 330/112 324/57 output voltage isadded to the initial voltage to initiate [5;] Illt. Cl. .1103! 19/00 aregenerative build up of the voltage i d to h [5 Field of Search ..330/112, 9, 1.00, 85,2; input The output voltage instantaneously increases324/57 325/486 until the prescribed difference exists between the inputand the output voltages. A reading of this volt- [56] References Citedage indicates the maximum voltage swing at the UNITED STATES PATENTSprescribed difference between the input and output volta es. 3,582,6756/1971 Jordon, Jr. et al. ..330/9 X g 3,496,407 2/1970 Entermann..315/27 4 Claims, 2 Drawing Figures TERM/NALS FOR B/AS/NG VOLTAGEBACKGROUND or THE INVENTION 1. Field of the Invention This inventionrelates to systems and methods for determining the maximum outputvoltage swing on operational amplifiers, and more specifically, tosystems and methods wherein a constant voltage is initially applied toan operational amplifier under test and the resultant output is added tothe constant voltage to initiate a regenerative action to drive theoperational amplifier to a measurable maximum output voltage with aprescribed difference between the input and output voltages of theoperational amplifier.

2. Description of the Prior Art High volume production of integratedcircuit operational amplifiers requires a method of quickly determiningthe operating characteristics of the amplifiers. Factors contributing tothe wide usage of operational amplifiers are the multitude of functions,e.g., summation, inversion, integration, and differentiation, whichoperational amplifiers may perform depending upon the characteristics ofthe input and feedback impedances. This versatility has resulted in anincreasingly wide application of operational amplifiers to fields suchas computer technology and telephony. However, commensurate withincreased usage is the need for correspondingly efficient methods ofdetermining the operational amplifiers operating characteristics.

One such characteristic is the maximum output voltage swing, that is,the maximum value of the output voltage corresponding to a specifiedamount of nonlinearity or difference between input and output voltageswhich occurs with the onset of saturation. A possible method fordetermining the maximum output voltage swing would comprise overdrivingthe amplifier under test with an input larger than necessary forsaturation. However, such a procedure would render difficult thedetermination of the maximum output at a specified difference in inputand output voltages, and internal biasing under overdrive conditionsmight result in an output voltage that is lower than the availablemaximum.

Another test sequence comprises measuring and comparing the values ofvoltage input and output when the input is initiated at some value belowthat necessary for the expected output and then increased incrementally.I-Iere, using program storage to initiate and control the incrementedapplications of test voltages, an excessive time is required fortesting, particularly if bipolar testing is required and if more thanone bias voltage is to be used. Also, even with the use of a test devicehaving program storage, the possibly numerous incremental steps mayrequire excessive storage facilities.

It may be thus appreciated that there is a need for a test system whichquickly determines the maximum output voltage on operational amplifiersfor a prescribed difference between the input and output voltages.

SUMMARY OF THE INVENTION The present invention contemplates systems andmethods for automatically driving an operational am- 5 plifier through avoltage swing to a maximum output,

and for efficiently monitoring the maximum output voltage at a specifieddifference or non-linearity between input and output voltages.

In view of the aforementioned fact that the maximum output voltage swingis determined at a specified difference in input and output voltages,the invention further contemplates systems and methods for presettingthe specified difference as a voltage and then utilizing this voltage ina regenerative feedback system for driving an operational amplifier tothe maximum output voltage swing with the difiference in input andoutput voltages corresponding to the preset difference in voltages.

More particularly, the invention may take the form of a feedback addersystem which is interposed between the output and input, terminals ofthe opera tional amplifier under test. The adding function of thefeedback system may be provided by a second operational amplifier.Initially, a constant voltage is applied to the second operationalamplifier to generate an output having the value of the specifiedvoltage difference. This voltage is fed by the feedback system as inputto the amplifier under test and the resulting output is added by thesecond operational amplifier to the output generated by the appliedconstant voltage, initiating a regenerative increase in the voltageapplied to the amplifier under test and driving the amplifier under testthrough its maximum output voltage swing. A voltage measuring orrecording device connected to the output of the amplifier under testgives a reading of the maximum voltage output at the specifieddifference in input and output voltages.

BRIEF DESCRIPTION OF THE DRAWINGS 7 FIG. 1 is a simplified schematicrepresentation of an operational amplifier which is to be driven tosaturation by a system which embodies the principles of this invention;and

FIG. 2 is a simplified schematic representation of an embodiment of asystem which may be used to practice the principles of this inventionand to drive an operational amplifier to a maximum voltage swing with aprescribed difference between the input and output voltages.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown in schematicform a conventional operational amplifier of the type which may betested by the systems and methods of the present invention, designatedgenerally by the numeral 10 and having input terminals 11 and 12 and anoutput terminal 13. The internal circuitry of the amplifier is such thatthe polarity of signals applied to terminal 11 is inverted by theamplifier and impressed on the output terminal. Signals applied toterminal 12 pass through the amplifier without inversion of polarity. Afeedback loop 17 containing a resistor 15 connects the output terminal13 to the input terminal 11 to hold the output terminal at zeropotential during non-operating periods. The gain of amplifier 10 mayhave a multitude of values, depending upon the ratio of the relativevalues of feedback resistor 15 and input resistor 14. For purposes ofexplanation, resistors 15 and 14 are equalvalued, so that amplifier isconfigured as a unity gain amplifier and signals applied to terminal 11pass through the amplifier with substantially no change in magnitude.

In the use of these amplifiers, there is a maximum voltage swing whichmay be utilized before the drop in voltage through the amplifier becomesexcessive, that is, there is a drift away from the linear relationshipbetween the input and output voltages. In utilization of such amplifiersin computing, telephone or other control systems, the amplifier mustproduce an output that is linearly related, within limits, to the input.Otherwise, the resulting output voltage is erroneous or non-predictableand cannot be used to accomplish the desired system functions.

Referring now to FIG. 2, there is shown an embodiment of the presentinvention wherein the operational amplifier 10 is connected in afeedback adder system for driving the operational amplifier towardssaturation, and through a voltage swing wherein the driving actionceases upon occurrence of a predetermined difference between the inputand output voltages.

A principal feature of this system is a second operational amplifier 20,hereinafter referred to as the feedback amplifier, utilized as a voltageadder. The amplifier 20, which may be of a type such as Philbrick P65 orP65AU or Fairchild A74lC, has a conventional configuration somewhatsimilar to that of amplifier 10, i.e., terminals 21 and 22 for invertingand non-inverting inputs, respectively, an output terminal 23, an inputresistor 24 in series with inverting input terminal 21, and a feedbackloop 27 containing a resistor 25 which connects the output terminal 23to the inverting input terminal 21. Resistors 24 and 25 are of equalvalue, configuring amplifier 20 as a unity gain amplifier with respectto inverting input terminal 21. This operational amplifier 20 performsan adding function in that a voltage which is present at terminal 13 isinverted by amplifier 20 and added to a voltage that is generated by aninput voltage impressed on terminal 22.

The output terminal 23 of the feedback amplifier 20 is coupled to theinverting input terminal 11 of amplifier 10 through a feedback loop 28and the input resistor 14. Also, the output terminal 13 of amplifier 10is connected to the inverting input terminal 21 of feedback amplifier 20through the input resistor 24.

As mentioned'previously, in the configuration shown in FIGS. 1 and 2,the input and feedback resistors 14 and 15 of amplifier 10 are of equalvalue, e.g., 100K, and resistors 24 and 25 associated with amplifier arealso of equal value, e.g., 10K. These amplifiers are thereforeconfigured as unity gain inverters with respect to signals applied toterminals 11 or 21, and are connected in a feedback circuit running fromthe output terminal 23 of the amplifier 20, over the feedback loop 28,the resistor 14, the input 11 of amplifier 10, the output 13 ofamplifier 10, and the resistor 24 connected to the input terminal 21 ofthe amplifier 20. It will be noted that a signal impressed over thisfeedback circuit is inverted by the amplifier l0 and then inverted backto the original polarity by the functioning of the second operationalamplifier 20.

As is characteristic of operational amplifiers, the gain associated withnon-inverting input terminal 22 of amplifier 20 is twice that forinverting input terminal 21. Since amplifier 20 is configured as a unitygain amplifier with respect to inverting input terminal 21 in thepresent instance, an input voltage applied to non-inverting terminal 22generates an output voltage of the same polarity, and of substantiallytwice the magnitude, as the input voltage.

The non-inverting input 22 of the feedback amplifier may be connected toa voltage divider network, designated generally by the numeral 30,comprised of resistors 31 and 32 and terminals 34, 35, and 36. When asmall constant voltage is applied to terminal 34, a part of thatvoltage, as determined by the values of the network resistors, isapplied through junction terminal 36 to the input terminal 22.

In view of the gain of 2 associated with input terminal 22, it isconvenient to use resistors 31 and 32 that are equal-valued, e.g., 10K,so that the value of the voltage applied to the input terminal 22through terminal 36 is one-half the value of the voltage applied toterminal 34. Thus, in response to the voltage applied to terminal 22,the feedback amplifier 20 generates an output voltage of the samepolarity, and of substantially the same magnitude, as the input voltageto terminal 34. This output voltage is applied through feedback loop 28as input to amplifier 10. The amplifier 10 inverts the input voltage andthe resulting output voltage is impressed upon and inverted by thefeedback amplifier 20 to the same polarity as the polarity of theconstant voltage applied to the terminal 34. The feedback amplifier 20combines or adds this inverted output with the constant voltage appliedto terminal 34, triggering a regenerative action which cumulativelyincreases the input to amplifier l0 and drives that amplifier towardsaturation. This regenerative action will continue until the drop involtage across the amplifier 10 is equal to the voltage added atterminal 34. In this situation, the amplifying section of theoperational amplifier 20 associated with the input terminal 21 no longerapplies an increasing voltage to be added to the constant voltageapplied to the terminal 34, and the regenerative cycling is disruptedand the system is stabilized.

Proper operation of the system requires that the amplifier 10 saturatebefore the feedback amplifier 20. One method of ensuring this is to usehigher bias voltages for the feedback amplifier than for the amplifier10. A typical arrangement of bias voltages is shown in FIG. 2, wherepins designated 38 and 39 supply, e.g., +15 and-l5v bias, respectively,to the feedback amplifier. The relatively lower biasing level foramplifier 10 is supplied by terminals 18 and 19; the exact value isdetermined by the specification for the particular amplifier 10 undertest.

It should be noted that the present invention permits testing in boththe positive and negative modes. Thus, application of a positiveconstant voltage to terminal 22 of amplifier 20 generates a negativeoutput in amplifier 10. Similarly, if a negative voltage is applied toterminal 22, a positive output is generated.

In utilizing the aforementioned system to measure the maximum outputvoltage swing, with predetermined limits of variation between the inputand output voltages, a typical test specification might require that theoutput of amplifier exceeds fibv when that amplifier is biased at :6.2vand the absolute value of the difference between input andoutputvoltages does not exceed 0.3v. What is required is that thedifference between input and output as applied to terminal 34 be equalto 0.3v and the. measured output voltage exceed 5.6V for the 6.2vbiasing level. Thus, measurement of the value of the. input voltage toamplifier 10 is not required. As shown in FIG. 2, a terminal 40 isprovided for connecting a device 41 to measure and/or record the valueof the output voltage. A reading of this output voltage at stabilizationgives the valueof the maximum output voltage swing, i.e., the maximumoutput voltage for the specified 0.3v difference in input and output.

Considering a typical operation under the above-outlined testspecification, application of +.3v to terminal 34 results in theapplication of l 5v to input terminal 22 of amplifier 20. Then +.3vappears on the output terminal 23 and is impressed over the feedbackloop28 to the input terminal 11 of amplifier 10. Amplifier 10 functions toinvert the voltage. and apply .3v on the output terminal 13, whereuponthis voltage is impressed through resistor 24 to the input terminal 21of amplifier 20. Amplifier thereupon inverts and adds this voltage tothe .3v voltage generated by the .l5v voltage impressed on inputterminal 22. This action is instantaneous and initiates the repetitiveaction of adding voltages generated by the input voltage impressed onthe terminal 22 to the inverted output voltages of amplifier 10 andapplying the added voltages over the feedback loop tothe. input terminal11 of amplifier 10. This regenerative action continues until there isa.3v drop in voltage impressed through the amplifier 10. At this time,the regenerative feedback action is just adding .3v, which is equal tothe drop across the amplifier. Thus, the voltage swing impressed on theinput terminal 11 ceases, and a reading maybe taken at terminal 13 togive the maximum voltage swing with the prescribed .3v variation ininput and output voltages on the amplifier 10.

The system may be used to measure the amplifier response to negativevoltages by merely changingthe polarity of the voltage supplied to theterminal 34. Further, the system may be used to test other than unitygain amplifiers, in which case the voltage applied to terminal 34 willhave to be related to the amplification characteristics of the amplifierso as to provide absolute readings that are meaningful.

It is to be understood that the above-identified embodiments are simplyillustrative of the principles of the invention and numerous othermodifications may be devised without departing from the spirit and scopeof the invention.

What is claimed is:

l. A system for determining the maximum; output voltage of a firstoperational amplifier upon the occurrence of a specified voltagedifference between input and output voltages thereof; wherein the firstoperational amplifier has an inverting input terminal and an outputterminal, which comprises:

a second operational amplifier having an inverting input terminal, anon-inverting input terminal, and an output terminal;

means for applying a voltage to said non-inverting input terminal ofsaid second operational amplifier so that a voltageequal to thespecifiedvoltage difference is developed at said output terminal of said secondoperational amplifier;

means for coupling said output terminal of said second operationalamplifier to the inverting input terminal of the first operationalamplifier;

means for coupling the output terminal of the first operationalamplifier to said inverting input terminal of said second operationalamplifier so that the output voltage of the first operational amplifieris added to the specified voltage difference developed at said outputterminal of said second operational amplifier thereby initiating aregenerative feedback action to continuously increase the voltageapplied to the inverting input terminal of the first operationalamplifier until the voltage dif ferencebetween the inverting inputterminal and the output terminal of the first operational amplifier isequal to the specifiedvoltage difference; and

means coupled to the output terminal of the first operational amplifierfor measuring the output voltage of the first operational amplifier todeter mine the maximum output voltage upon the occur rence of thespecified voltage difference.

2. A system for determining the maximum output voltage of a firstoperational amplifier as set forth in claim 1 wherein said applyingmeans further comprises a voltage divider network having a junctioncoupled to said non-inverting input terminal of said second operationalamplifier and having an input terminal so thatapplying a voltage equalto the specified voltage difference to said input terminal of saidnetwork produces an output voltage from said second operational amplifier equal to the specified voltage difference.

3. In a system for determining the maximum output voltage of a firstoperational amplifier which is obtainable without exceeding apredetermined voltage drop between an inverting input terminal and anoutput terminal of the first operational amplifier;

a second operational amplifier having an inverting input terminal, anon-inverting input terminal and an output terminal, the output terminalof the first operational amplifier being coupled to said inverting inputterminal of said second operational amplifier;

a voltage divider network coupled to said. non-inverting input of saidsecond operational amplifierand having an input terminal, so thatapplying a preselected voltage equal to the predetermined voltage dropto said input terminal of said voltage divider network produces avoltage at said output terminal of said second operational amplifierequal to the predetermined voltage drop solely in response to theapplying of said preselected voltage;

means for coupling said output terminal of said second operationalamplifier to the inverting input terminal of the first operationalamplifier so that a regenerative feedback action is established to drivethefirst operational amplifier toward saturation, said regenerativeaction automatically ceasing when the voltage drop between the invertinginput terminal and the output terminal of the first operationalamplifier equals said preselected volt age; and

means coupled to the output terminal of the first operational amplifierfor measuring the output voltage to determine when said regenerativefeedback action has ceased thereby determining the maximum outputvoltage from the first operational amplifier corresponding to apredetermined voltage drop between the inverting input terminal and theoutput terminal.

4. A method of determining the maximum output voltage of an operationalamplifier corresponding to a specified voltage difference between inputand output voltages, wherein the first operational amplifier has aninverting input terminal and an output terminal, which comprises:

introducing a voltage equal to the specified voltage difference into aregenerative feedback circuit coupling the output terminal to the inputterminal of the operational amplifier;

increasing the voltage applied to the input terminal of the operationalamplifier by said regenerative feedback circuit in steps equal to thesum of the specified voltage difference and the voltage present at theoutput terminal of the operational amplifier;

stabilizing the voltage applied to the input terminal when the voltagedrop between the input terminal and the output terminal equals thespecified voltage difference; and

measuring the output voltage of the operational am-

1. A system for determining the maximum output voltage of a firstoperational amplifier upon the occurrence of a specified voltagedifference between input and output voltages thereof; wherein the firstoperational amplifier has an inverting input terminal and an outputterminal, which comprises: a second operational amplifier having aninverting input terminal, a non-inverting input terminal, and an outputterminal; means for applying a voltage to said non-inverting inputterminal of said second operational amplifier so that a voltage equal tothe specified voltage difference is developed at said output terminal ofsaid second operational amplifier; means for coupling said outputterminal of said second operational amplifier to the inverting inputterminal of the first operational amplifier; means for coupling theoutput terminal of the first operational amplifier to said invertinginput terminal of said second operational amplifier so that the outputvoltage of the first operational amplifier is added to the specifiedvoltage difference developed at said output terminal of said secondoperational amplifier thereby initiating a regenerative feedback actionto continuously increase the voltage applied to the inverting inputterminal of the first operational amplifier until the voltage differencebetween the inverting input terminal and the output terminal of thefirst operational amplifier is equal to the specified voltagedifference; and means coupled to the output terminal of the firstoperational amplifier for measuring the output voltage of the firstoperational amplifier to determine the maximum output voltage upon theoccurrence of the specified voltage difference.
 2. A system fordetermining the maximum output voltage of a first operational amplifieras set forth in claim 1 wherein said applying means further comprises avoltage divider network having a junction coupled to said non-invertinginput terminal of said second operational amplifier and having an inputterminal so that applying a voltage equal to the specified voltagedifference to said input terminal of said network produces an outputvoltage from said second operational amplifier equal to the specifiedvoltage difference.
 3. In a system for determining the maximum outputvoltage of a first operational amplifier which is obtainable withoutexceeding a predetermined voltage drop between an inverting inputterminal and an output terminal of the first operational amplifier; asecond operational amplifier having an inverting input terminal, anon-inverting input terminal and an output terminal, the output terminalof the first operational amplifier being coupled to said inverting inputterminal of said second operational amplifier; a voltage divider netwoRkcoupled to said non-inverting input of said second operational amplifierand having an input terminal, so that applying a preselected voltageequal to the predetermined voltage drop to said input terminal of saidvoltage divider network produces a voltage at said output terminal ofsaid second operational amplifier equal to the predetermined voltagedrop solely in response to the applying of said preselected voltage;means for coupling said output terminal of said second operationalamplifier to the inverting input terminal of the first operationalamplifier so that a regenerative feedback action is established to drivethe first operational amplifier toward saturation, said regenerativeaction automatically ceasing when the voltage drop between the invertinginput terminal and the output terminal of the first operationalamplifier equals said preselected voltage; and means coupled to theoutput terminal of the first operational amplifier for measuring theoutput voltage to determine when said regenerative feedback action hasceased thereby determining the maximum output voltage from the firstoperational amplifier corresponding to a predetermined voltage dropbetween the inverting input terminal and the output terminal.
 4. Amethod of determining the maximum output voltage of an operationalamplifier corresponding to a specified voltage difference between inputand output voltages, wherein the first operational amplifier has aninverting input terminal and an output terminal, which comprises:introducing a voltage equal to the specified voltage difference into aregenerative feedback circuit coupling the output terminal to the inputterminal of the operational amplifier; increasing the voltage applied tothe input terminal of the operational amplifier by said regenerativefeedback circuit in steps equal to the sum of the specified voltagedifference and the voltage present at the output terminal of theoperational amplifier; stabilizing the voltage applied to the inputterminal when the voltage drop between the input terminal and the outputterminal equals the specified voltage difference; and measuring theoutput voltage of the operational amplifier to determine when saidapplied voltage is stabilized thereby determining the maximum outputvoltage of the operational amplifier corresponding to the specifiedvoltage difference between input and output voltages.